Humidity controller

ABSTRACT

A breathing assistance apparatus adapted to deliver humidified gases at a desired level of humidity to a patient including a humidifier and a heated conduit is disclosed. The humidifier includes a controller which determines the flow rate of the gases and then determines the required power input to the humidifier to deliver the gases to the patient at the required patient humidity. This means the need for external sensors is dispensed with and thus the apparatus is simple and less bulky.

TECHNICAL FIELD

This invention relates to breathing assistance apparatus, particularlybut not solely, for supplying optimal humidity temperature of gases to apatient to assist the patient's breathing.

BACKGROUND ART

A number of methods are known in the art for assisting a patient'sbreathing. Continuous Positive Airway pressure or CPAP involves theadministration of air under pressure to a patient, usually by a nasalmask. It is used in the treatment of snoring and Obstructive Sleep Apnea(OSA), a condition characterised by repetitive collapse of the upperairway during inspiration. Positive pressure splints the upper airwayopen, preventing its collapse. Treatment of OSA with nasal CPAP hasproven to be both effective and safe, but CPAP is difficult to use andthe majority of patients experience significant side effects,particularly in the early stages of treatment.

Upper airway symptoms adversely affect treatment with CPAP. Mucosaldrying is uncomfortable and may awaken patients during the night.Rebound nasal congestion commonly occurs during the following day,simulating a viral infection. If untreated, upper airway symptomsadversely affect rates of CPAP use.

Increases in nasal resistance may affect the level of CPAP treatmentdelivered to the pharynx, and reduce the effectiveness of treatment. Anindividual pressure is determined for each patient using CPAP and thispressure is set at the mask. Changes in nasal resistance affect pressuredelivered to the pharynx and if the changes are of sufficient magnitudethere may be recurrence of snoring or airway collapse.

Such symptoms can also occur in a hospital environment where a patientis on a respirator. Typically in such situations the patient isintubated. Therefore the throat tissue may become irritated and inflamedcausing both distress to the patient and possible further respiratoryproblems.

A number of methods may be employed to treat such upper airway symptoms,including pharmacologic agents to reduce nasal disease, or heating thebedroom. One most commonly employed method is humidification of theinspired air using an in line humidifier. Two types of humidifier arecurrently used. Cold passover humidifiers rely on humidifying the airthrough exposure to a large surface area of water. While they are cheap,the humidity output is low at high flows, typically 2 to 4 mg\L absolutehumidity at flows above 25 L/min. The output is insufficient to preventmucosal drying. Heated water bath humidifiers are more efficient, andproduce high levels of humidity even at high flow rates. They areeffective at preventing upper airway mucosal drying, prevent increasesin nasal resistance, and are the most reliable means of treating upperairway symptoms.

Any of these active systems will have, to some degree or other,condensation (or rain out) in the tubing connecting the humidifier tothe patient. The degree of condensation is strongly dependent on theambient temperature, being much greater for greater differences betweenthe ambient temperature and the gas temperature. The formation of largequantities of water in the breathing tubing causes considerableinconvenience to the patient, may accelerate cooling of the gas, mayeventually occlude the tubing, or may be expelled into the patient.Also, the patient may experience discomfort, when breathing gases aredelivered at temperatures widely divergent from that of the ambienttemperature. Excessive condensation also results in inefficient usage ofthe water in the humidifying chamber.

In a hospital environment, where the ambient temperature of theatmosphere within the hospital environment is controlled by airconditioning for example, the required temperature for the humidifiedgases supplied by the apparatus may be controlled within set temperatureparameters that are sufficiently close to the ambient temperature toprevent condensation within the conduit. However it is still necessaryto have good control over the temperature and humidity of gases as theyare actually supplied to the patient.

In the home care environment in which a user requires to use humidifyingapparatus at home, the range of ambient and gas temperatures may wellexceed that of the hospital environment. In the home care environment,the user will usually wear a face mask which is connected to end of theconduit and such a humidifier may be used in the home environment forthe treatment of breathing and sleep apnea disorders and/or inconjunction with ventilators or CPAP devices. In addition, non activehumidifiers are commonly employed utilising the known pass overhumidification technique.

In U.S. Pat. No. 5,640,951 issued to Fisher and Paykel a heated conduitfor a humidified breathing assistance apparatus is disclosed whichincludes a temperature probe at the end of a heated conduit. By heatingthe conduit the problems relating to condensation in the conduit may beovercome. However in order to implement closed loop control over thetemperature of the supplied gases (and therefore the power input to theconduit heater element), it is necessary to measure the temperature asclose to the point at which it is supplied as possible. The temperatureprobe and its associated wiring included for this purpose make theattachment to the face mask or intubated patient bulky and thereforemore uncomfortable for the patient. Therefore it would be advantageousif a heated conduit for a humidified breathing assistance apparatuscould be implemented without the need for a temperature probe at the endof the conduit. It would also be advantageous to have some indication,when the conduit heater is energised, that it is operating correctly.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a breathingassistance apparatus which goes some way to overcoming theabovementioned disadvantages or which at least provides the public orindustry with a useful choice.

Accordingly in a first aspect the invention consists in a breathingassistance apparatus adapted to deliver humidified gases at a desiredlevel of humidity or at a desired temperature to a patient using openloop control comprising:

a humidifier having an electrical input power and capable of humidifyingsaid gases up to a level of humidity prior to delivery to said patient,said level of humidity depending on said input power to said,

and

a controller or processor configured or programmed to:

(a) determine a parameter relating to the flow rate of said gasesthrough said apparatus;

(b) determine based on at least said parameter the required electricalpower input to said humidifier to deliver said gases to said patient ata level of humidity or at a temperature substantially similar to saiddesired level of humidity or said desired temperature;

(c) supply as said input power to said humidifier a level of powersubstantially similar to said determined power input to said humidifier.

In a second aspect the invention consists in a breathing assistanceapparatus adapted to deliver humidified gases at a desired level ofhumidity or at a desired temperature to a patient comprising:

humidifier having an electrical input power capable of humidifying saidgases up to a level of humidity prior to delivery to said patient, saidlevel of humidity depending on said input power to said humidifier,

conduit for conveying said humidified gases from said humidifier to saidpatient, and

conduit heater having an electrical input power, and being associatedwith said conduit wherein the gases flowing through said conduit areheated either directly or indirectly by said conduit heater whereby thelevel of heating depending on said input power to said conduit heater;

controller or processor which supply said input power to said humidifierand said conduit heater, and providing a control output indicative ofsaid conduit heater being correctly connected to said controller orprocessor and capable of operating in according within predefinedlimits; and

a connector means to electrically connect said controller or processorand said conduit heater and including an indicator in use connected tosaid control output, wherein when said said conduit heater beingcorrectly connected to said controller or processor and capable ofoperating in according within predefined limits said controller orprocessor energising said indicator.

In a third aspect the invention consists in a method of deliveringhumidified gas at a desired level of humidity or at a desiredtemperature to a patient using an open loop controlled humidifiercomprising the steps of:

(a) determining a parameter relating to the flow rate of said gasthrough said humidifier;

(b) determining based on at least said parameter the required electricalpower to said humidifier to deliver said gas to said patient at a levelof humidity or at a temperature substantially similar to said desiredlevel of humidity or said desired temperature; and

(c) supplying a level of power to said humidifier substantially similarto said determined power.

In a fourth aspect the invention consists in a method of connecting aconduit heater within a conduit to a humidifier comprising the steps:

providing an electrical connection between said conduit heater and saidhumidifier; and

indicating whether conduit heater being correctly connected and capableof operating in according within predefined limits.

In a fifth aspect the invention costs in a breathing assistanceapparatus adapted to deliver humidified gas at a desired level ofhumidity or at a desired temperature to a patient using open loopcontrol comprising:

humidifier having an electrical input power and capable of humidifyingsaid gas up to a level of humidity prior to delivery to said patient,said level of humidity depending on said input power to said humidifier,

means for determining a parameter relating to the flow rate of said gasthrough said apparatus;

means for determining based on at least said parameter the requiredelectrical power input to said humidifier to deliver said gas to saidpatient at a level of humidity or at a temperature substantially similarto said desired level of humidity or said desired temperature;

means for supplying as said input power to said humidifier a level ofpower substantially similar to said determined power input to saidhumidifier.

To those skilled in the art to which the invention relates, many changesin construction and widely differing embodiments and applications of theinvention will suggest themselves without departing from the scope ofthe invention as defined in the appended claims. The disclosures and thedescriptions herein are purely illustrative and are not intended to bein any sense limiting.

BRIEF DESCRIPTION OF THE DRAWINGS

One preferred form of the present invention will now be described withreference to the accompanying drawings in which;

FIG. 1 is a illustration of a respiratory humidifier system,

FIG. 2 is a illustration of the humidifier base of the respiratoryhumidifier system of FIG. 1,

FIG. 3 is a block diagram of the control system which controls thehumidifier in the preferred embodiment of the present invention,

FIG. 4 is a flow diagram of the algorithm used to control the heaterwire within the respiratory conduit,

FIG. 5 is an example of how the heater plate temperature varies overtime, when the pressure is controlled constant,

FIG. 6 is a graph of heater plate power against flow rate, and

FIG. 7 is a graph of conduit heater element power and flow rate.

DETAILED DESCRIPTION OF THE INVENTION

Whether used in a hospital environment or in a home care environment,the present invention will generally have associated two main pieces ofapparatus. Firstly an active humidifier which controls the temperatureof a heater plate heating a body of water to achieve a desiredtemperature and humidity of the gases being humidified. Secondly atransport conduit from the humidifier to the patient is also required,which is preferably heated to reduce condensation, or “rain out”.

Referring to FIG. 1 a humidifying apparatus as might be used in ahospital generally referenced 1 is shown. The apparatus comprises a body2 containing heating means comprising a heating plate 20 having anelectric heating element therein or in thermal contact therewith andcontrol means for example electronic circuitry which may include amicroprocessor for controlling the supply of energy to the heatingelement. The body 2 is removably engageable with a. humidifying chamber3 which contains water for humidifying gases. Referring to FIG. 2 whichshow the humidifier apparatus in more detail, the humidifying chamber 3has edges which engage with collar 24 on the humidifier apparatus. Thegases to be humidified may be a mixture of air, oxygen and anaestheticfor example which are supplied to the chamber through a gases inlet 4.This might be connected to a ventilator, or in the case of CPAP therapya CPAP blower. A gases outlet 5 is also provided and the gases outlet 5is connected to the conduit 6 (FIG. 1) which conveys humidified gases toa remote destination such as an intubated patient at the end 7 of theconduit. Alternatively, the end 7 of the conduit may have a gas maskattached thereto, which mask is used to cover a nose and/or mouth of auser so as to supply humidified gases to the user for breathing, as inthe delivery of CPAP therapy. The humidifier heater plate 20 has atemperature transducer 8 which is in electrical connection with theelectronic control circuitry in body 2 of the apparatus so that thecontrol means monitors the temperature of the heating plate.

A heating element 10 is provided within the conduit 6 to help preventcondensation of the humidified gases within the conduit. Suchcondensation is due to the temperature of the walls of the conduit beingclose to the ambient temperature, (being the temperature of thesurrounding atmosphere) which is usually lower than the temperature ofthe humidified gases within the conduit. The heater element iseffectively replaces the energy lost from the gases through conductionand convection during transit through the conduit. Thus the conduitheater element ensures the gases delivered are at an optimal temperatureand humidity.

The present invention provides a means of controlling at least theheater plate and preferably also the conduit heater element without theneed for any sensors, either in the humidifier chamber or positioned inthe conduit. This is achieved by estimating the rate of flow of gasesthrough the humidifier using parameters already available to thecontroller. For a given humidifier an appropriate level of power canthen be determined to apply to the heater plate to achieve the desiredtemperature of gases delivered to the patient. Additionally this may beused to provide a more appropriate level of energisation at this conduitheater element. This not only saves the cost of the extra sensors butalso allows the apparatus connected to the end of the conduit to besimpler and lighter.

In the preferred embodiment of the present invention the controller 100,shown in FIG. 3, uses a range of inputs to control both the power 108supplied to the heater plate 110 as well as the power 114 supplied tothe conduit heating element 116 (if present). In certain applications itmay also be used to provide control instructions to auxiliary apparatussuch as a blower fan. Using an internal algorithm 106 the controller 100estimates the power 108 to supply to the humidifier heater plate 110 toachieve a given humidity and or temperature of gases at the top of thehumidifier chamber alternatively (or estimates the temperature toachieve a given power). It then uses a second algorithm 102 to estimatethe required power 114 to supply to the conduit heater element 116 andthe humidifier heater plate 110 to achieve optimal temperature and/orhumidity of the gases delivered to the patient 118.

Referring to FIG. 4, when the humidifier starts up the controllerexecutes a supervisory algorithm, which controls the heater plate and ifpresent the conduit heater element. Initially 128 the heater plate iscontrolled to a temperature of 40° C. and the conduit heater element maybe energised with a duty cycle of for example 50%. The heater platetemperature (or alternatively the power supplied to the heater plate) isthen monitored 130 until it settles to a stabilised level. Effectively awindow 132 is superimposed over the heater plate temperature profile 134of which an example is shown in FIG. 5. When the profile 134 (over theentire period of the window 132) fits within the bounds of the window132, it is effectively considered to have stabilised. Once this hasoccurred the controller enters a calculation stage.

Firstly, it calculates the flow rate of the gases 136 using any one of anumber of methods which will be described later.

Secondly knowing the rate of flow of the gases the algorithm thencalculates the required heater plate power 138 (alternatively heaterplate temperature) to achieve a desired temperature/humidity of gases(alternatively heater plate power). A relationship has been empiricallydetermined using a humidifier and a heated conduit such as that asdescribed in U.S. Pat. No. 5,640,951, the contents of which areincorporated herein by reference. The actual relationship for any otherarrangement would either have to be empirically determined byexperimentation or theoretically calculated. For a desired temperatureof gases exiting the humidifier of for example 37° C. the relationshipbetween the power supplied to the heater plate (P_(HP)), the rate offlow of gases (F_(gas)) and the ambient temperature (T_(amb)) is graphedin FIG. 6. From this an approximate general algebraic equation has beenextrapolated which the controller can use to determine an approximatelevel of power to apply to the heater plate:

P _(HP)=(−0.1239×T _(amb)+5.383)×F _(gas)+(−0.3112×T _(amb)+10.738)

Thirdly the algorithm calculates the required power input to the conduitheater wire 140 to deliver a desired temperature of the gases to thepatient. With gases flowing at a known rate of flow it is possible tocalculate the resultant temperature of the gases once they have flowedthrough a conduit of known characteristics surrounded by the atmosphereat a known or assumed ambient temperature. Thermal characteristics ofthe conduit will either be known or can be calculated byexperimentation. This relationship is based off empirical data using ahumidifier and a heated conduit such as that as described in U.S. Pat.No. 5,640,951. The actual relationship for any other arrangement wouldeither have to be empirically determined by experimentation ortheoretically calculated. With a conduit entry gas temperature of 37° C.and a temperature of gases delivered to the patient of 40° C., therelationship between the flow rate of the gases (F_(gas)), the powerinput to the conduit heater element (P_(c)), the ambient temperature(T_(amb)) is graphed in FIG. 7. This is extrapolated to a generalalgebraic expression:

P _(c)=(−0.0005*T _(amb)+0.0169) F _(gas) ²−[10⁻⁵ *T _(amb) ³−0.0042*T_(amb) ²+0.2189*T _(amb)−3.0075]F _(gas)−1.0169*T _(amb)+38.956

Practically this relationship can be simplified whereby P_(c) isdependent only on T_(amb). This is an acceptable approximation for theconduit heater element, as it is not as crucial as the heater plate.

Once the heater plate and conduit heater element have been appropriatelyenergised, the controller continues to monitor 142 the system for anychanges in the variables. The main reason for this is to avoid thermalovershoot i.e. where the flow drops suddenly, the temperature of gasescan become dangerously high.

In order to monitor effectively, two methods are used. Firstly the flowrate is monitored and secondly the change in flow rate (with respect totime) is also monitored. The first 144 is to allow the system to respondto any changes in the system. The second 146 is a fast response systemin order to avoid thermal overshoot. Effectively where either P_(HP) orT_(HP) is controlled constant, monitoring the other variable gives anindication of any change in flow, or any other variable which requires arecalculation.

In order to monitor the flow a variable x (defined as P_(HP)/T_(HP)),which is closely related to the flow rate, is constantly calculated andmonitored. If it goes up there is a 30 minute delay before thecontroller initiates a recalculation, to avoid spurious readings andunnecessary calculations. If it goes down there is a 30 second delaybefore the controller recalculates, to avoid any possibility of thedelivered gases being, even transiently, too hot.

Where large step changes occur the controller needs to react quickly. Insuch cases it will reset to initial conditions to wait until the systemstabilises again, as any calculations in the interim would be pointless.To achieve this dx/dt is calculated and monitored. While a negativevalue is more dangerous, any deviation over a certain value will resetthe controller.

In an alternative embodiment of the present invention the expectedheater plate temperature is calculated using

T _(HP)=−7.3319*Ln(F _(gas))+63.655

and if the actual heater plate temperature deviates by more than 5° C.then the program recalculates the required powers.

Thus in summary controller carries out the following steps:

-   1) Estimates the rate of flow of gases keeping all variables    constant 136.-   2) Estimate the required heater plate power/temperature to achieve a    specified temperature/humidity of gases in the humidification    chamber 138.-   3) Calculate the power input to the heater wire to achieve a desired    output temperature 140.

It will be appreciated that a greater level of power will be supplied tothe conduit heater element if:

-   i) the rate of flow of the gases reduces,-   ii) the ambient temperature decreases,-   iii) the differential between ambient and gases temperature    increases.

It will also be appreciated that the heater plate temperature could becontrolled to a set valve (using closed loop control) as opposed topower. In this case the power supplied would be monitored as a measureof system stability. Furthermore where relationships are expressedalgebraically they could equally be stored in look-up tables. Firstpreferred embodiment of flow estimation

Generally when used in a hospital setting a humidifier such as thatdescribed in the present invention will be used in conjunction with arespirator to supply humidified gases to an incubated patient, orpossibly using a respiratory mask. As such the humidifier will operateeffective independently of the respirator and therefore must make all ofits control decisions based on only the sensors contained therein. Inthe preferred embodiment of the present invention the flow rate of thegases passing through the humidification chamber can first be estimatedby comparing the power input required 108 for the humidifier heaterplate to the measured temperature 112 of the heater plate. In effect thehigher the rate of flow of gases the larger the amount of power requiredby the heater plate in order to achieve a given heater platetemperature. Thus for a given system the relationship between power toheater plate and flow rate for a given heater plate temperature caneither be determined empirically or theoretically calculated. Againusing a humidifier and a heated conduit such as that as described inU.S. Pat. No. 5,640,951 the following empirically determinedrelationship applies:

$F_{gas} = \frac{\begin{matrix}{{- ( {0.831 - {0.0049*T_{amb}}} )} +} \\\sqrt{{abs}{{( {0.831 - {0.0049*T_{amb}}} )^{2} - ( {4*( {{0.00004*T_{amb}} - 0.0057} )*( {( {14.348 - {0.25*T_{amb}}} ) - P_{HP}} )} )}}}\end{matrix}}{2*( {{0.0004*T_{amb}} - 0.0057} )}$

where P_(HP) is the power applied to the heater plate to achieve a givenheater plate temperature in steady state of 50° C., T_(amb) is theambient temperature and F_(gas) is the gas flow rate.

It will be appreciated this method is more appropriate in the hospitalcare environment where the ambient temperature can be assured with ahigh degree of confidence.

Second Preferred Embodiment of Flow Estimation

In the homecare environment the present invention will often be employedin conjunction with a continuous positive airway pressure (CPAP) deviceor such other breathing apparatus which will include a fan such as thatdescribed in U.S. Pat. No. 6,050,260, the contents of which areincorporated herein by reference. It will be appreciated that in suchapplications it may be possible to connect the controllers of thevarious devices together in an arrangement such that data may be readilyexchanged. In such cases the rate of flow of the gases may be estimateddirectly from information available either from the fan or, whereprovided, a flow sensor.

In this embodiment of the present invention the flow is estimated basedon the loading of the fan. Generally the fan will be controlled to runat a specified speed and therefore deliver a constant pressure output.The flow rate of the gases will depend on the restrictions in the flowpath. In turn in order to maintain the specified speed a certain powerinput will be required for the fan. Therefore an algebraic relationshipbetween the actual gas flow rate and the power input to the fan can bedeveloped for a fan of known characteristics. This relationship mayeither be determined empirically by experimentation or theoreticallycalculated using specified motor characteristics.

A number of methods are known in the art for determining the loading ona motor from the supply it draws. The simplest such method would be tofirstly meter the current drawn 148 from the fan 150, as indicated inFIG. 3. The current 148 is the input to the conduit heater elementcontroller 102 where either an algebraic relationship or a look up tableis used to determine the flow rate of the gases.

For example in U.S. Pat. No. 5,740,795, the contents of which are herebyincorporated herein by reference, a method is disclosed using both motorvoltage and current to estimate the flow rate. While this represents onemethod, as mentioned above, it will be appreciated that other methods,such as based on just current, will be equally applicable.

Third Preferred Embodiment of Flow Estimation

As mentioned in the second embodiment that in certain cases a flowsensor may already be provided in the gas flow path. This being thecase, the gas flow rate 152 can be extracted directly from the flowsensor 154 and used as an input to the humidifier controller 100, asindicated in FIG. 3. This is then used directly in the conduit heaterelement controller 102 to determine the power to apply to the heaterplate 110 and conduit heater element 116 according to the algorithmshown in FIG. 4 and described earlier.

Heater Wire Adaptor

In order to connect the conduit heater element to the power supply inthe humidifier, an adaptor cable is required. In the preferredembodiment of the present invention, the adaptor 200 includes anindicator 202 to indicate whether the conduit heater element isoperating correctly, when the adaptor is plugged in, as shown in FIG. 1.

The humidifier controller continually detects for the conduit heaterelement and determines whether it is operating correctly. It does thisby energising the conduit heater element intermittently, and if theexpected current results it energises 204 the indicator (e.g. an LED).

The present invention as described in the foregoing provides a novelmethod and apparatus for controlling the heater plate temperature in ahumidifier for supplying humidified gases to a patient under respiratorytherapy. This has the advantage of removing external sensors making thesystem simpler, cheaper and lighter. Similarly it may also allow foreffective control over energisation of the conduit heater element,ensuring the system as a whole operates correctly as well as being asefficient as possible.

1-32. (canceled)
 33. A breathing assistance apparatus comprising: aheater configured to heat water to humidify gases, the heater furtherconfigured to be in a fluid communication with a conduit configured todeliver the gases to a patient; and a controller configured to: cause aninitial power level to be supplied to the heater to maintain a heatertemperature at an initial temperature level; determine a flow rate ofthe gases based on an output of a flow sensor; based on the flow rate ofthe gases, determine a power level to heat the gases to a selectedtemperature or humidify the gases to a selected humidity and cause thedetermined power level to be supplied to the heater; monitor a change inthe flow rate of the gasses; in response to determining that the changein the flow rate of the gases satisfies a first threshold but does notsatisfy a second threshold, redetermine the power level to heat thegases to the selected temperature or to humidify the gases to theselected humidity and cause the redetermined power level to be suppliedto the heater; and in response to determining that the change in theflow rate satisfies the second threshold, cause the initial power levelto be supplied to the heater, the second threshold being indicative of alarger change in the flow rate of the gases than the first threshold.34. The apparatus of claim 33, wherein the controller is furtherconfigured to: in response to determining that 1) the change in the flowrate of the gases satisfies the first threshold but does not satisfy thesecond threshold and 2) the change in the flow rate of the gasesindicates an increase in the flow rate of the gases, delayredetermination of the power level to heat the gases to the selectedtemperature or to humidify the gases to the selected humidity for afirst period of time.
 35. The apparatus of claim 34, wherein thecontroller is further configured to: in response to determining that 1)the change in the flow rate of the gases satisfies the first thresholdbut does not satisfy the second threshold and 2) the change in the flowrate of the gases indicates a decrease in the flow rate of gases, delayredetermination of the power level to heat the gases to the selectedtemperature or to humidify the gases to the selected humidity for asecond period of time shorter than the first period of time.
 36. Theapparatus of claim 33, further comprising a temperature sensorconfigured to measure the heater temperature, wherein the controller isfurther configured to: determine an expected heater temperature based onthe flow rate of the gasses; and in response to determining that theexpected heater temperature deviates from the measured heatertemperature by a threshold temperature, redetermine the power level toheat the gases to the selected temperature or to humidify the gases tothe selected humidity and cause the redetermined power level to besupplied to the heater.
 37. The apparatus of claim 33, wherein the flowrate of the gases depends at least in part on one or more restrictionsin a flow path configured to accommodate a flow of the gases.
 38. Theapparatus of claim 33, wherein the controller is further configured todetermine a conduit heater power level to heat the gases flowing throughthe conduit and cause the conduit heater power level to be supplied tothe conduit heater.
 39. The apparatus of claim 38, wherein thecontroller is further configured to increase the conduit heater powerlevel in response to at least one of: increase in the flow rate of thegases, decrease in ambient temperature, or increase in a differencebetween the ambient temperature and a temperature of the gases.
 40. Abreathing assistance apparatus comprising: a heater configured to heatwater to humidify gases, the heater further configured to be in a fluidcommunication with a conduit configured to deliver the gases to apatient; and a controller configured to: cause an initial power level tobe supplied to the heater to maintain a heater temperature at an initialtemperature level; estimate a flow rate of the gases; based on theestimated flow rate of the gases, determine a power level to heat thegases to a selected temperature or humidify the gases to a selectedhumidity and cause the determined power level to be supplied to theheater; monitor a change in the flow rate of the gasses; in response todetermining that the change in the flow rate of the gases satisfies afirst threshold but does not satisfy a second threshold, redetermine thepower level to heat the gases to the selected temperature or to humidifythe gases to the selected humidity and cause the redetermined powerlevel to be supplied to the heater; and in response to determining thatthe change in the flow rate satisfies the second threshold, cause theinitial power level to be supplied to the heater, the second thresholdbeing indicative of a larger change in the flow rate of the gases thanthe first threshold.
 41. The apparatus of claim 40, further comprising afan configured to blow the gases through a flow path, wherein thecontroller is configured to estimate the flow rate of the gases based onloading of the fan.
 42. The apparatus of claim 41, wherein the flow rateof the gases depends at least in part on one or more restrictions in theflow path.
 43. The apparatus of claim 40, further comprising atemperature sensor configured to measure ambient temperature, whereinthe controller is configured to estimate the flow rate of the gasesbased on the power level to heat the gases to the selected temperatureor humidify the gases to the selected humidity and the ambienttemperature.
 44. The apparatus of claim 40, further comprising atemperature sensor configured to measure the heater temperature, whereinthe controller is configured to estimate the flow rate of the gasesbased on a ratio of the determined power level to heat the gases to theselected temperature or to humidify the gases to the selected humidityand the heater temperature measured by the temperature sensor.
 45. Theapparatus of claim 40, wherein the controller is further configured to:in response to determining that 1) the change in the flow rate of thegases satisfies the first threshold but does not satisfy the secondthreshold and 2) the change in the flow rate of the gases indicates anincrease in the flow rate of the gases, delay redetermination of thepower level to heat the gases to the selected temperature or to humidifythe gases to the selected humidity for a first period of time; and inresponse to determining that 1) the change in the flow rate of the gasessatisfies the first threshold but does not satisfy the second thresholdand 2) the change in the flow rate of the gases indicates a decrease inthe flow rate of gases, delay redetermination of the power level to heatthe gases to the selected temperature or to humidify the gases to theselected humidity for a second period of time shorter than the firstperiod of time.
 46. The apparatus of claim 40, further comprising atemperature sensor configured to measure the heater temperature, whereinthe controller is further configured to: determine an expected heatertemperature based on the flow rate of the gasses; and in response todetermining that the expected heater temperature deviates from themeasured heater temperature by a threshold temperature, redetermine thepower level to heat the gases to the selected temperature or to humidifythe gases to the selected humidity and cause the redetermined powerlevel to be supplied to the heater.
 47. The apparatus of claim 40,wherein the controller is further configured to determine a conduitheater power level to heat the gases flowing through the conduit andcause the conduit heater power level to be supplied to the conduitheater.
 48. The apparatus of claim 47, wherein the controller is furtherconfigured to increase the conduit heater power level in response to atleast one of: increase in the flow rate of the gases, decrease inambient temperature, or increase in a difference between the ambienttemperature and a temperature of the gases.
 49. A method of operating abreathing assistance apparatus, the method comprising, by a controllerof the breathing assistance apparatus: causing an initial power level tobe supplied to a heater of the breathing assistance apparatus tomaintain a heater temperature at an initial temperature level, theheater configured to heat water to humidify gases; estimating a flowrate of the gases; based on the estimated flow rate of the gases,determining a power level to heat the gases to a selected temperature orhumidify the gases to a selected humidity and causing the determinedpower level to be supplied to the heater; monitoring a change in theflow rate of the gasses; in response to determining that the change inthe flow rate of the gases satisfies a first threshold but does notsatisfy a second threshold, redetermining the power level to heat thegases to the selected temperature or to humidify the gases to theselected humidity and causing the redetermined power level to besupplied to the heater; and in response to determining that the changein the flow rate satisfies the second threshold, causing the initialpower level to be supplied to the heater, the second threshold beingindicative of a larger change in the flow rate of the gases than thefirst threshold.
 50. The method of claim 49, wherein estimating the flowrate of the gases comprises determining loading of a fan of thebreathing assistance apparatus.
 51. The method of claim 49, whereinestimating the flow rate of the gases is based on the power level toheat the gases to the selected temperature or humidify the gases to theselected humidity and ambient temperature.
 52. The method of claim 49,wherein estimating the flow rate of the gases comprises determining aratio of the determined power level to heat the gases to the selectedtemperature or to humidify the gases to the selected humidity and theheater temperature measured by a temperature sensor of the breathingassistance apparatus.
 53. The method of claim 49, wherein estimating theflow rate of the gases comprises determining the flow rate of the gasesbased on an output of a flow sensor of the breathing assistanceapparatus.